1. Technical Field of the Invention
The present invention relates to an optical head for detecting a focusing error signal in optical information read and write apparatuses, wherein both land and groove of recording medium are employed for reading and writing by optical beam from the optical head.
2. Description of the Prior Art
The astigmatic aberration method is often employed conventionally in order to obtain the focusing error signal, because it is easily combined with the push pull method for obtaining the tracking error signal.
A conventional optical head according to the astigmatic aberration method is shown in FIG. 7. The light from semiconductor laser 101 is collimated by collimator lens 102. Then, about 50% of the light passes through beam splitter 103. Further, the light is focused on optical disk 105 by objective lens 104. The light reflected by optical disk 105 passes through objective lens 104. Then, about 50% of the reflected light is reflected by beam splitter 103, and is detected by photo-detector 108 through cylindrical lens 106 and convex lens 107.
A plan view of cylindrical lens 106 is shown in FIG. 8. As shown in FIG. 8, the angle between the axis 110 of cylindrical lens 106 and the radial direction of optical disk 105 is xcex8. Here, xcex8 is 45xc2x0.
Photo-detector 108 and beam spot thereon are shown in FIGS. 9A, 9B, and 9C, wherein the focused positions of the beam are different from each other. Photo-detector 108 comprises four light detecting portions 111 to 114 on which the light passing through cylindrical lens 106 is detected. The left to right direction in FIG. 8 is the radial direction of optical disk 105 and the down to up direction in FIG. 8 is the tangential direction of optical disk 105, while the left to right direction in FIG. 9 is the tangential direction of optical disk 105 and the down to up direction in FIG. 9 is the radial direction of optical disk 105, due to cylindrical lens 106.
In FIG. 9A, the major axis of elliptic beam spot 115 is directed from the lower left to the upper right, because optical disk 105 is positioned nearer to objective lens 4 than the focusing point. In FIG. 9B, the beam spot is circular, because optical disk 105 is positioned just on the focusing point. In FIG. 9C, the major axis of beam spot 115 is directed from the upper left to the lower right, because optical disk 105 is positioned farther from objective lens 4 than the focusing point.
The focusing error signal FE equals to ((V111+V114)xe2x88x92(V112+V113)), when the outputs from light detecting portions 111 to 114 are V111 to V114, respectively. FE becomes negative, zero, and positive, in FIGS. 9A, 9B, and 9C, respectively.
Further, the tracking error signal TE for the push pull method equals to ((V111+V112)xe2x88x92(V113+V114)). Furthermore, the read out signal RF equals to (V111+V112+V113+V114).
Another conventional optical head which detects focus error signal, according to the astigmatic aberration method is shown in FIG. 10. This optical head is disclosed in Applied Optics/Vol.32, No.29/Oct. 10 ,1993, pp 5789 to 5796. Light beam from semiconductor laser 101 is collimated by collimator lens 102. Then, about 50% of the light passes through beam splitter 103. Further, the light is focused on optical disk 105 by objective lens 104. The light beam reflected by optical disk 105 passes through objective lens 104. Then, about 50% of the reflected light is reflected by beam splitter 103. Then, the beam reflected by beam splitter 103 is divided into 50% transmission beam and 50% reflection beam by beam splitter 109. The 50% transmission beam is detected by photo-detector 108a through cylindrical lens 106a and convex lens 107a, while the 50% reflection beam is detected by photo-detector 108b through cylindrical lens 106b and convex lens 107b. 
A plan view of cylindrical lens 106a is shown in FIG. 11A, while a plan view of cylindrical lens 106b is shown in FIG. 11B. The angle between the axes 110a and 10b of cylindrical lenses 106a and 106b and the radial direction of optical disk 105 is xcex8. Here, xcex8 is 45xc2x0.
Photo-detector 108a and beam spot thereon are shown in FIGS. 12A, 12B, and 12C, while photo-detector 108b and beam spot thereon are shown in FIGS. 12D, 12E, and 12F. Photo-detector 108a comprises four light detecting portions 111a to 114a on which the 50% transmission beam from beam splitter 109 becomes beam spot 115a, while photo-detector 108b comprises four light detecting portions 111b to 114b on which the 50% reflection beam from beam splitter 109 becomes beam spot 115b. 
The left to right direction in FIGS. 11A and 11B is the radial direction of optical disk 105 and the down to up direction in FIGS. 11A and 11B is the tangential direction of optical disk 105, while the left to right direction in FIGS. 12A to 12F is the tangential direction and the down to up direction is the radial direction, due to cylindrical lenses 106a and 106b. Beam spot 115a and beam spot 115b are mirror symmetrical in respect to the down to up direction.
In FIG. 12A, the major axis of elliptic beam spot 115a is directed from the lower left to the upper right, because optical disk 105 is positioned nearer to objective lens 104 than the focusing point. In FIG. 12B, the beam spot 115a is circular, because optical disk 105 is positioned just on the focusing point. In FIG. 12C, the major axis of elliptic beam spot 115a is directed from the upper left to the lower right, because optical disk 105 is positioned farther from the objective lens 104 than the focusing point.
In FIG. 12D, the major axis of elliptic beam spot 115b is directed from the upper left to the lower right, because optical disk 105 is positioned nearer to objective lens 104 than the focusing point. In FIG. 12E, the beam spot 115b is circular, because optical disk 105 is positioned just on the focusing point. In FIG. 12F, the major axis of elliptic beam spot 115b is directed from the lower left to the upper right, because optical disk 105 is positioned farther from the objective lens 104 than the focusing point.
The focusing error signal FEa detected by photo-detector 108a equals to ((V111a+V114a)xe2x88x92(V112a+V113a)), when the outputs from light detecting portions lila to 114a are V111a to V114a, respectively. Similarly, the focusing error signal FEb detected by photo-detector 108b equals to ((V111b+V114b)xe2x88x92(V112b+V113b)), when the outputs from light detecting portions 111b to 114b are V111b to V114b, respectively. Here, FEa becomes negative, zero, and positive, in FIGS. 12A, 12B, and 12C, respectively, while FEb becomes positive, zero, and negative, in FIGS. 12D, 12E, and 12F, respectively. Therefore, the focusing error FE obtained by the optical head as shown in FIG. 10 becomes (FEaxe2x88x92FEb) which is negative in FIGS. 12A and 12D, zero in FIGS. 12B and 12E, and positive in FIGS. 12C and 12F.
Further, the tracking error signal TEa detected by photo-detector 108a for the push pull method equals to (V111a+V112a)xe2x88x92(V113a+V114a), while the tracking error signal TEb detected by photo-detector 108b equals to ((V111b+V112b)xe2x88x92(V113b+V114b)). Therefore, the tracking error signal TE detected by the optical head as shown in FIG. 10 becomes (TEa+TEb).
Furthermore, the read out signal RF is calculated on the basis of the outputs from 108a and 108b. 
The read out signal RFa obtained by photo-detector 108a is (V111a+V112a+V113a+V114a), while the read out signal RFb obtained by photo-detector 108b is (V111b+V112b+V113b+V114b). Therefore, the readout signal RF obtained by the optical head as shown in FIG. 10 becomes (RFa+RFb). Thus, the FE signal, TE signal, ,and RF signal are obtained by the optical head as shown in FIG. 10. Further, it has been demonstrated that the interference between the FE signal and the TE signal is smaller in the optical head as shown in FIG. 10 than in the optical head as shown in FIG. 7.
Further, the so called land/groove recording for high density optical disk is explained, referring to FIGS. 13A and 13B. In the land recording, light beam 116 is irradiated on to land 117 which is a concave portion as shown in Figure in 13A of a pre-grooved disk. Likewise, in the groove recording, light beam 116 is irradiated on to groove 118 which is a convex portion as shown in FIG. 13B of the pre-grooved disk.
The focusing error signal from the land is equal to that from the groove, only when the axis of cylindrical lens is directed strictly to 45xc2x0 from the radial direction of the optical disk. However, The angle xcex8 is deviated from 45xc2x0, due to errors in the manufacturing and assembling steps of optical light detecting portions and head.
The relations between focusing error signal and defocus distance are shown in FIGS. 14A, 14B, and 14C. Here, the focusing error signal FE is normalized by the readout signal RF. Further, the focusing signal from land (land focusing signal 119) is indicated by solid line, and the groove focusing signal 120 is indicated by dotted line. Further, sensitivity of focusing error signal is defined herein as an absolute value of the inclination of linear plot in the focusing error vs. defocus relation. The focusing error sensitivity is the same for land and groove, as shown in FIG. 14B, when the angle xcex8 is 45xc2x0. On the other hand, the sensitivity of the land focusing error 119 (land sensitivity) becomes greater than the groove sensitivity, as shown in FIG. 14A, when the angle xcex8 is greater than 45xc2x0. On the contrary, the land sensitivity becomes smaller than the groove sensitivity, as shown in FIG. 14C, when the angle xcex8 is smaller than 45xc2x0.
The land focusing error actually has positive off-set at zero defocus, while the groove focusing error actually has negative off-set at zero defocus. However, these off-sets can be compensated completely by electronic circuits, although they are inevitable in principle.
Further, the focusing error signals FEa due to the 50% transmission beam from beam splitter 109 of the optical head as shown in FIG. 10 are shown in FIGS. 15A, 15B,and 15C. The horizontal axis is the defocus distance, and the vertical axis is FEa/RFa. Further, land focusing error 119a is indicated by solid line, and groove focusing error 120a is indicated by dotted line.
Likewise, the focusing error signals FEb due to the 50% reflection beam from beam splitter 109 of the optical head as shown in FIG. 10 are shown in FIGS. 15D, 15E,and 15F.
As shown in FIGS. 15B and 15E, the focusing error sensitivity is the same for land and groove, when the angle xcex8 is 45xc2x0. On the other hand, as shown in FIGS. 15A and 15D, the land sensitivity becomes greater than the groove sensitivity, when the angle xcex8 is greater than 45xc2x0. On the contrary, as shown in FIGS. 15C and 15F, the land sensitivity becomes smaller than the groove sensitivity, when the angle xcex8 is smaller than 45xc2x0.
The normalized focusing error signal FE/RF is ((FEa/RFa)xe2x88x92(FEb/RFb))/2 for the optical head as shown in FIG. 10. In this optical head, the angle xcex8 is independently deviated from 45xc2x0, and the above-defined FE/RF signal for the land recording may possibly be greater or smaller than the FE/RF for the groove recording.
The land focusing error actually has positive off-set at zero defocus, while the groove focusing error has actually negative off-set at zero defocus, in FIGS. 15A, 15B,and 15C. Likewise, the land focusing error actually has negative off-set at zero defocus, while the groove focusing error has actually positive off-set at zero defocus, in FIGS. 15D, 15E, and 15F. However, these off-sets can be compensated completely by electronic circuits, although they are inevitable in principle.
Therefore, the conventional optical heads as shown in FIGS. 7 and 10 has a disadvantage that a gain of focusing servo circuit can not be properly adjusted. Concretely, when the gain is optimized for the lower sensitivity side of the focusing error signal, the focusing servo circuit begins oscillating for the higher sensitivity side. On the other hand, when the gain is optimized for the higher sensitivity side of the focusing error signal, the gain of the focusing servo circuit becomes too small to eliminate the focusing error residue for the lower sensitivity side. Accordingly, the focusing servo becomes instable, during access operations when the optical head traverses the pre-grooved tracks. In short, the conventional optical heads are not suitable for the land/groove recording.
Therefore, an object of the present invention is to provide an optical head suitable for the land/groove recording, wherein the land focusing error can be made equal to the groove focusing error.
In the present invention, a light beam reflected by a recording medium is diffracted by a holographic element. Then, the xc2x11st order diffracted light beams are detected by a photo-detector. The holographic element functions as cylindrical lenses for the xc2x11st order diffracted light beams. The axis of the first cylindrical lens for the +1st order light beam is directed at +45xc2x0 from the direction normal to the track direction, while the axis of the second cylindrical lens for the xe2x88x921st order light beam is directed at xe2x88x9245xc2x0 from the direction normal to the track direction. Further, the second focusing error signal on the basis of the xe2x88x921st order diffracted light beam is subtracted from the first focusing error signal on the basis of the +1st order diffracted light beam in order to obtain the focusing error signal.
According to the optical head of the present invention, wherein the focusing error signal is generated by the difference between the focusing error signals obtained by the xc2x11st order diffracted light beams, the focusing error sensitivities for both land and groove are the same, even when the axis of the holographic element is deviated from xc2x145xc2x0, due to errors in the manufacturing and assembling. In other words, the sensitivities of focusing error signal become the same for land and groove, regardless of manufacturing error of optical parts and assembling error of optical head, because the focusing error signal is generated on the basis of the difference between the focusing error signals detected by the xc2x11st order diffracted beams. Therefore, the focusing servo on land/groove recording system is stabilized, because the servo gains are optimized simultaneously for both land and groove.